This application claims priority to German application DE 199 04 603.4, filed Feb. 5, 1999, herein incorporated by reference.
1. Field of the Invention
The present invention relates to maleic anhydride copolymers containing amine oxide groups and to their use as dispersants for pigments or fillers.
2. Description of the Related Art
In accordance with the prior art it is common to use, as dispersants, polyacrylic esters having acidic and/or basic groups, which may also be in salt form, which can be prepared by polymerizing corresponding monomeric acrylic esters, such as butyl acrylate, acrylic acid, 2-hydroxyethyl acrylate and its alkoxylation products, and other monomers having vinylic double bonds, such as styrene or vinylimidazole, for example (see for example EP-B-0 311 157). There are also descriptions, however, of how it is possible by transesterification reactions on alkyl polyacrylates, by replacing the alkyl group in the manner of a polymer-analogous reaction by alcohols or amines, to produce dispersants of this kind (see for example EP-B-0 595 129, DE-C-39 06 702, EP-A-0 879 860). Advantages of the polymer-analogous reaction regime are said to lie in the narrower molecular weight distribution and absence of disruptive or toxic monomer fractions. In this way it is also said to be possible to produce polymers which are not available by direct polymerization owing to the lack of industrial availability of corresponding polymerizable monomers. Such alcoholyses or aminolyses often require the use of, in general, strongly acidic catalysts such as, for example, sulfuric acid or para-toluenesulfonic acid, or metal salts such as, for example, titanates, which at the reported reaction temperatures of up to 200xc2x0xc2x0C. result not only in unwanted side reactions, which cause an increase in molecular weight, but also in often strongly colored products, whose coloring means that they are poorly suited or even totally unsuitable for demanding dispersion tasks.
In a very similar way, use is also made of polymers containing diesters of free-radically polymerizable dicarboxylic acids, such as dialkyl maleates, for example, as a basis for preparing dispersants, as set out, for example, in WO 96/14347. That document describes first of all a transesterification of the alkyl dicarboxylate with a hydroxy-functional polyoxyalkylene followed by a subsequent copolymerization with a further vinylically unsaturated monomer. A disadvantage here again is that the transesterification step, in the presence of preferably acidic catalysts, requires reaction temperatures of approximately 160xc2x0xc2x0C., which again entail the abovementioned disadvantages in respect of the possible use of the dispersants obtained in this way. The subsequent copolymerization is dependent on the addition of large amounts of costly organic peroxides as initiator in order to obtain sufficient conversions and reproducible copolymers. Despite this, and especially when using monomers having weight-average molecular weights of more than 500 g/mol, there still remain considerable fractions of unpolymerized monomer in the reaction mixture in unreacted form, which can also no longer be removed economically. In addition to the economic disadvantages, this is particularly undesirable owing to the potential toxic properties inherent in the majority of free-radically polymerizable vinyl compounds. Furthermore, the products have generally undergone further severe darkening after the copolymerization. This again places great restrictions on possible applications.
With the prior art dispersants described above it is, moreover, impossible to meet (fully) the diverse performance requirements made in particular in the sector of high-quality industrial coatings, such as, for example, the OEM finishing of automobiles. For instance, time and again, disadvantages which are in some cases mutually contradictoryxe2x80x94for example, absence of compatibility with various binders, adequate activity only at very high levels of added dispersant, excessive dispersion times, levels of achievable pigmentation which are too low, insufficient development of color strength, constancy of viscosity and shade (especially after storage), or flocculation and aggregation (including that observed after a time delay) owing to excessively weak stabilization of the pigments by the dispersantxe2x80x94prevent their broad use.
An object of the present invention is therefore to overcome a large number of the above disadvantages and to provide novel dispersants which are easy to prepare and which in particular feature extraordinarily good performance properties.
It has surprisingly been found that it is possible to obtain novel dispersant which are superior to the prior art and do not have the above mentioned disadvantages.
These are maleic anhydride copolymers containing amine oxide groups, which are obtainable by reacting at least about 1 mol % of the anhydride groups of maleic anhydride copolymers with diamines from the group consisting of
a) HR1Nxe2x80x94R2xe2x80x94NR3R4 
xe2x80x83where
R1 is hydrogen or a monovalent hydrocarbon radical having 1 to 24 carbon atoms,
R2 is a divalent alkylene radical having 2 to 24 carbon atoms, and
R3 and R4 are aliphatic and/or alicyclic alkyl radicals having 1 to 12, preferably 1 to 6, carbon atoms, which can be identical or different
and subsequently carrying out oxidation with the formation of amine oxide groups which are attached by way of monoamide groups and/or imide groups and are of the following general formula (as the skilled worker is well aware, the wavy lines are intended to indicate that, hereinbelow, this representation relates merely to a section of a polymer compound) 
Preference is given to maleic anhydride copolymers containing amine oxide groups, in which a further portion of the anhydride groups has been reacted with alcohols and/or amines from the group consisting of
b) saturated or unsaturated, alicyclic or cyclic monohydroxy and/or monoamine compounds of the general formula
HOxe2x80x94R1 or HR2Nxe2x80x94R1, respectively,
xe2x80x83where
R1 is a hydrocarbon radical having 2 to 24 carbon atoms, and
R2 is hydrogen or a monovalent hydrocarbon radical having 1 to 24 carbon atoms,
with the formation of corresponding monoester, monoamide or imide structures, respectively.
Preference is likewise given to maleic anhydride copolymers containing amine oxide groups, in which a further portion of the anhydride groups has been reacted with
c) polyoxyalkylene compounds of the general formula
Hxe2x80x94(Oxe2x80x94C2H4xe2x88x92aR3a)b(Oxe2x80x94CcH2c)dOxe2x80x94R4,
xe2x80x83where
R3 are identical or different alkyl radicals or alkylene radicals having 1 to 24 carbon atoms or are substituted or unsubstituted phenyl radicals having up to 24 carbon atoms, and
R4 is hydrogen or a saturated or unsaturated, optionally branched or cyclic or aromatic hydrocarbon radical having up to 24 carbon atoms, which can, if desired, also include heteroatoms from the group consisting of O, S, N, P, Si, F, Cl, Br, and I,
a=0 to 3,
b=0 to 100,
c=2 to 12,
d=0 to 100,
and the sum (b+d)=2 to 200,
the sequence of the polyoxyalkylene segments (Oxe2x80x94C2H4xe2x88x92aR3a)b and (Oxe2x80x94CcH2c)d being arbitrary
with the formation of corresponding monoester structures.
Preference is likewise given to maleic anhydride copolymers containing amine oxide groups, in which a further portion of the anhydride groups has been reacted with
d) polyoxyalkylene compounds of the general formula
H2Nxe2x80x94R5xe2x80x94(Oxe2x80x94C2H4xe2x88x92aR3a)b(Oxe2x80x94CcH2c)dOxe2x80x94R6,
xe2x80x83where
R3 are identical or different alkyl radicals or alkylene radicals having 1 to 24 carbon atoms or are substituted or unsubstituted phenyl radicals having up to 24 carbon atoms,
R5 is a divalent alkylene radical having 2 to 6 carbon atoms, and
R6 is hydrogen or a saturated or unsaturated, optionally branched or cyclic or aromatic hydrocarbon radical having up to 24 carbon atoms, which can, if desired, also include heteroatoms from the group consisting of O, S, N, P, Si, F, Cl, Br, and I,
a=0 to 3,
b=0 to 100,
c=2 to 12,
d=0 to 100,
and the sum (b+d)=2 to 200,
the sequence of the polyoxyalkylene segments (Oxe2x80x94C2H4xe2x88x92R3a)b and (Oxe2x80x94CcH2c)d being arbitrary
with the formation of corresponding monoamide and/or imide structures.
The index a here can adopt different values in one polyoxyalkylene. This is intended to express the fact that suitable polyoxyalkylenes can be either, for example, homopolymers of ethylene glycol, copolymers of ethylene glycol and 1,2-propylene glycol, or else a multiple copolymer of more than two monomers, such as for example ethylene glycol, 1,2-propylene glycol and 1,2-butylene glycol. Irrespective of this, the index c can also adopt different values in one polyoxyalkylene, so that, for example, multiple copolymers can be built up additionally with 1,4-butylene glycol and the copolymers can have a random or block structure.
As comonomer(s), the maleic anhydride copolymers of the invention containing amine oxide groups may include at least one further, different monomer having vinylic double bonds, preferably a monomer from the group consisting of styrene, alkylated styrene compounds, (meth)acrylic acid, (meth)acrylic esters, alkyl vinyl ethers, vinyl acetate, and itaconic esters or mixtures of monomers from this group.
Preference is given to the use of styrene as comonomer, the molar ratio of maleic anhydride to styrene being between about 1:1 and about 1:3.
The maleic anhydride copolymers of the invention containing amine oxide groups preferably have a molecular weight Mw of from about 1000 to about 100,000, preferably from 2500 to 50,000, g/mol.
Depending on desired properties, the carboxylic acid groups present in the polymer can be neutralized to carboxylate salts.
The invention additionally provides for the use of the maleic anhydride copolymers containing amine oxide groups as dispersants for pigments or fillers, especially for preparing pigment concentrates, the dispersant being homogenized together with the pigments and/or fillers to be dispersed, if desired in the presence of organic solvents and/or water, if desired with binders and, if desired, with conventional coatings auxiliaries.
The invention likewise provides for the use of the copolymers of the invention for preparing a coating composition, a binder being dispersed together, if desired, with a solvent, with pigments and/or fillers, with the dispersant and, if desired, with auxiliaries.
The invention therefore provides a new class of dispersing additives having unexpectedly prominent properties.
The formula scheme below is to be seen as an example for the purpose of illustrating the reaction sequence and the structures formed in the course thereof: 
The skilled worker is well aware that, of course, through the use of compounds which contain more than just one functional group capable of nucleophilic substitution at anhydride groups, such as, for example, dihydroxy or trihydroxy compounds by means of intermolecular reaction at two or more anhydride groups, which therefore belong to different maleic anhydride copolymers, it is possible for there to be an increase in molecular weight as a result of a crosslinking reaction between at least two maleic anhydride copolymer chains. This fact can be utilized in a targeted manner, and, by way of the amount of polyfunctional compound added, the molecular weight of the overall molecule can be controlled within wide ranges in order to establish specific properties, such as the viscosity, for example.
The dispersing propertiesxe2x80x94long known to the skilled workerxe2x80x94of the maleic anhydride copolymers and their derivatives obtainable by alcoholyses or aminolyses, as are described, for example, inter alia, in JP 62 183 845, WO 97/15382 and EP-0 747 457, can be decisively improved by introducing amine oxide groups attached by way of monoamide and/or imide groups. Furthermore, this can be achieved with retention of the original diversity of structural variation possibilities of the maleic anhydride copolymers, which, as the skilled worker is well aware, are at least equal in their diversity to the abovementioned alcoholysis products and aminolysis products of alkyl acrylate polymers, by virtue of reaction with a very wide variety of nucleophils, but which offer distinct advantages in performance terms.
The fact that even a comparatively minor modification of the maleic anhydride copolymers, of from 1 mol % per anhydride group, with amine oxide groups of the above general formula which are attached by way of monoamide and/or imide groups, brings with it a marked improvement in the dispersing properties is surprising, since there is next to no knowledge of any dispersion-enhancing property of amine oxides for pigments and fillers. In this case it is found that amine oxides apparently have a particularly good capacity for interacting with a wide variety of pigment surfaces and so ensuring extremely reliable adsorption of the dispersant on the pigment surface. Amine oxides therefore act as particularly effective examples of what are known as xe2x80x9canchor groupsxe2x80x9d, as was described quantitatively long ago for carboxylic acid groups, for example, (for comparison, see L. Dintenfass, JOCCA, 41 (1958) 125); an overview of further anchor groups, the various mechanisms, and further literature is given in J. D. Schofield, Polymeric Dispersants, in Handbook of Coatings Additives, Vol. 2, 15 Edition (L. J. I. Calbo, Ed.), Marcel Dekker, New York (1992) 71.
Amine oxides themselves, however, according to the prior art, are described only generally in the form of and/or in combination with betaine-type compounds (xe2x80x9cinner saltsxe2x80x9d) as dispersion promoters, such as in JP 63 095 277 (for aluminum in printing inks), U.S. Pat. No. 4,486,405 (for metal oxides in cosmetic formulations), DE-B-25 46 810 (for fillers in epoxy systems), and DE-B-16 44 193 (after-treatment of azo dyes).
The maleic anhydride copolymers used as the basic raw material are available commercially (from Leuna-Harze GmbH, D-06236 Leuna) or can easily be obtained by methods common in the prior art, by free-radical polymerization of maleic anhydride with at least 100 mol % of a further, different monomer having vinylic double bonds, preferably a monomer from the group consisting of styrene, alkylated styrene compounds, (meth)acrylic acid, (meth)acrylic esters, alkyl vinyl ethers, vinyl acetate and itaconic esters, or mixtures of monomers from this group. Styrene as comonomer is particularly suitable when the molar ratio of maleic anhydride to styrene is between 1:1 and 1:3. Intense research into such polymers has been carried out since the nineteen forties and continuously thereafter, as shown for example by S. D. Baruah, N. C. Laskar, J. Appl. Polym. Sci. (1996), 60, 649 and U.S. Pat. No. 4,180,637.
The reaction of the anhydride groups with the specified amino and/or hydroxy compounds can be carried out either with the addition of solvents and catalysts or without additional solvents and catalysts. In any individual case, however, the use of basic catalysts such as tertiary amines of low nucleophilicity is advantageous. As a function of the chosen temperature and, with particular advantage, utilizing a temperature gradient, it is possible to establish the preferential formation of monoamides and imides, with the monoamides (addition reaction) forming at room temperature whereas the synthesis of the imides (condensation reaction) necessitates temperatures preferably of 80xc2x0xc2x0C. and above depending on the reaction component, solvent and experimental setup: 
The amine oxides of the invention can be prepared in accordance with methods known from the literature by means of simple oxidation with aqueous hydrogen peroxide solution at moderate temperatures. This reaction has no adverse effect on the other structural elements and functional groups that are present in the molecule in accordance with the invention. The reaction is exemplified below using a maleimide derivative of the invention: 
The skilled worker is well aware that, depending on the reaction conditions, and in the presence of water, it is likewise possible for there to bexe2x80x94at least partialxe2x80x94formation of the analogous amine oxide hydrates of the amine oxides of the invention. Their possible presence in the dispersants of the invention therefore requires no further specific mention and is, accordingly, sufficiently clarified and described with the use of the term xe2x80x9camine oxidexe2x80x9d.